1. Field of the Invention
This invention relates to copolymers of maleic anhydride and alkyl vinyl ethers prepared in benzene solvent, and, more particularly, to a process of rendering such copolymers substantially benzene-free, and to products produced thereby.
2. Description of the Prior Art
High molecular weight copolymers of maleic anhydride and alkyl vinyl ethers, e.g. C.sub.1 -C.sub.5 alkyl vinyl ethers, such as methyl vinyl ether, are prepared advantageously by precipitation polymerization in benzene solvent because of its very low chain transfer effect, in which the monomers are soluble but the copolymer is insoluble. Such copolymers, and their half-esters, hydrolyzed free acid and salts, find wide commercial use in adhesives, coatings, pharmaceuticals, hairsprays, thickeners, protective colloids, detergents and denture adhesives. However, benzene is not desired in the final product, it can be removed by hydrolysis of the product in water followed by azeotropic distillation. Benzene is more difficult to remove from the anhydride form of the polymer. Generally, such removal procedures have involved drying the benzene-containing copolymers at relatively high temperatures and for long periods of time. Unfortunately, the remaining benzene content of the anhydride copolymer still is about 1-3% by weight.
U.S. Pat. No. 4,532,320 describes a process for removal of physiologically unacceptable solvents such as benzene from polymers containing anhydride groups, such as copolymers of maleic anhydride and methyl vinyl ether. This process comprises heating the dry copolymer powder in an aliphatic hydrocarbon such as nonane or decane at a temperature which is at or above the glass transition temperature of the copolymer, e.g., at 161.degree. C. for a treatment mixture of equal parts of nonane and decane (see Example 7 of the patent). However, the final benzene content in the treated copolymer was 0.03 wt. %, or 300 ppm, which is still an objectional amount. One ppm or less is considered benzene-free.
Accordingly, an object of this invention is to provide a process of rendering copolymers of maleic anhydride and alkyl vinyl ethers prepared in benzene solvent substantially benzene-free in the anhydride form.
A particular object herein is to provide such copolymers having benzene present in an amount of less than 1 ppm and which are otherwise acceptable from a physiological standpoint.